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Thermostat and Magnetic Clutch Systems

  • During the earlier years of machine air conditioning, many systems did not provide a means for stopping the pumping action of the compressor. A solid pulley was installed on the compressor crankshaft, which resulted in compressor operation anytime the engine, was operating. The only time the compressor could be stopped was when the belt was removed. Even with the air conditioning controls in the "OFF" position during cold weather operation, a slight amount of cold air would be given off by the evaporator
  • Today, manufacturers are turning more and more to the thermostat-controlled system with a magnetic clutch.

Thermostat Control

The opening and closing of electrical contacts in the thermostat are controlled by a movement of a temperature-sensitive diaphragm or bellows. The bellows has a capillary tube connected to it which has been filled with refrigerant. The capillary tube is positioned so that it may have either the cold air from the evaporator pass over it or may be connected to the tail pipe of the evaporator.

In either position, evaporator temperature will affect the temperature-sensitive compound in the capillary tube by causing it to contract as the evaporator becomes colder. The contraction of the gas will cause the bellows to contract. This separates the electrical points and breaks the electrical circuit to the compressor clutch, which stops compressor operation.

Now the evaporator begins to warm which, in turn, gas in the capillary tube to expand. The bellows will also expand, moving the electrical circuit to the compressor clutch, energizing it and bringing the compressor into operation again. THis cycle is repeated for as long as the air conditioning is being used.

The thermostatic switch is made from a pivoting frame attached to the bellows. Movement of the bellows during expansion and contraction cause the frame to pivot. Springs control and counteract the movement. Half of the electrical contacts are connected to the frame, the other half are mounted to the switch, but insulated from the metal parts.


The distance the contacts must travel and the spring pressure must be overcome by the expanding gas in the capillary tubes and bellows determine at what degree of evaporator the contacts will close to complete the electrical circuit.

In all thermostats, the spring tension and point spacing may be varied by the operator to regulate evaporator cooling for comfort. Temperature is controlled by rotating a cam (via a knob control) which increases or decreases spring tension of a pivoting point.

Part Identification - Thermostat


Relative Note
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